A dramatic rise in osteoporosis occurs in women after menopause. Its cause is linked to increased osteoclasts and decreased osteoblast activation due to increased inflammatory bone resorbing cytokines. In the USA today, 8 million women have osteoporosis and 15 million have osteopenia, placing them at increased risk for osteoporosis and bone fractures. Since hormone replacement therapy (HRT) causes adverse effects, alternative safe therapies against bone loss are badly needed. Although various dietary factors are associated with the bone loss, the roles of n-6 (plant) and n-3 (fish) oil intake have not been fully studied as yet in an ovariectomized (OVX) animal model. Our recent studies have revealed anti-inflammatory effects of fish oil against osteoporosis in OVX mice. Bone loss measured by DEXA was significantly less in both sedentary and treadmill exercised n-3 fatty acid-fed OVX mice compared to corn oil-fed OVX mice. The decreased bone mineral density (BMD) in corn oil-fed OVX mice is correlated with elevated RANKL expression in T cells when compared to OVX mice fed n-3 fatty acids. We hypothesize that the OVX groups will have less osteoclast activation when supplemented with n-3 fatty acids due to alteration in membrane fatty acids resulting in a decrease in IL- 1Beta-IL-6, and TNFalpha levels and RANKL expression. We now propose to test this hypothesis by comparing the effect of sham and OVX in middle-aged (12mo) osteoporosis-prone C57BL/6J mice fed n-6 and n-3 diets with and without exercise. We will carry out the following: 1) Measure changes in BMD and histomorphometry of bones by feeding a corn oil diet and a diet containing deodorized fish oil with 18% eicosapentaenoic acid (EPA) and 12% docosahexaenoic acid (DHA). 2) Determine the combined effects of n-3 fatty acids with treadmill or voluntary wheel exercise on BMD in 12 month old OVX mice. 3) Measure changes in bone marrow and lymphocyte subsets, proinflammatory cytokines, serum leptin and IGF-1 levels, and RANK, RANKL and NF-KappaB expression in vitro with n-3 and n-6 fatty acids. In summary, the proposed studies will investigate the dietary and molecular mechanisms of bone loss during aging in OVX mice and its attenuation by low-cost n-3 lipid supplements with moderate exercise as alternatives to HRT.